The present invention relates to a control system of an engine, particularly to a control system of an engine where compression self-ignition combustion is performed when an operating state of the engine is within a predetermined compression self-ignition range, and spark-ignition combustion is performed when the operating state of the engine is within a predetermined spark-ignition range.
Conventionally, engines which perform compression-ignition combustion triggered by pre-mixture compression self-ignition (Homogeneous-Charge Compression Ignition: HCCI) are known. Such engines which perform the HCCI combustion use a so-called internal exhaust gas recirculation (EGR) system for opening an exhaust valve in a predetermined valve-open period during an intake operation to reverse burned gas back into a combustion chamber from an exhaust port, so as to increase a temperature of a mixture gas inside the combustion chamber.
However, an operating range of the engine where the HCCI combustion can stably be performed is limited to a range where an engine speed and an engine load are relatively low (HCCI range). Therefore, in the engines which perform the HCCI combustion, the HCCI combustion is performed when the operating state of the engine is within the HCCI range, and when the operating state of the engine is within an operating range where the engine speed and the engine load are relatively high, which is outside the HCCI range, SI (Spark-Ignition) combustion is performed.
For example, in a case where a vehicle operator steps onto an accelerator pedal while the operating state of the engine is within the HCCI range and the engine is operated by the HCCI combustion, and a request load on the engine is thus increased and the operating range of the engine is shifted from the HCCI range to an operating range where the SI combustion is performed (SI range), an operation mode of the exhaust valve is required to be switched from a mode for the HCCI combustion to a mode for the SI combustion and combustion control parameters (e.g., an injection amount of fuel and an injection timing thereof) are also required to be switched from set values for the HCCI combustion to set values for the SI combustion.
In the switch of the combustion mode between the HCCI combustion and the SI combustion as above, an internal EGR amount in a transition period for the operation mode of the exhaust valve to switch is insufficient for the HCCI combustion, whereas it is excessive for the SI combustion, which may cause unstable combustion and/or knocking.
In this regard, for example, JP4737103B discloses a control system of an engine. With this control system, when switching the combustion mode, a pre-mixture gas is first formed substantially uniformly inside a cylinder, fuel is injected by a fuel injector on a compression stroke to form a stratified mixture gas concentrating around an ignition plug, and then the stratified mixture gas is ignited to combust (stratified spark-ignition combustion), so as to induce self-ignition of the pre-mixture gas. By assisting the self-ignition of the pre-mixture gas with the spark-ignition combustion of the stratified mixture gas around the ignition plug as above, unstable combustion can be prevented and a sufficient engine torque can be obtained by the self-ignition combustion.
However, in the stratified spark-ignition combustion described above, a generation amount of nitrogen oxide increases when a rich mixture gas formed around the ignition plug is combusted, and thus, emission performance degrades. Additionally, fuel consumption degrades due to the increase of the fuel injection amount.
Therefore, it is desirable to shorten as much as possible the period of performing the stratified spark-ignition combustion when switching the combustion mode; however, with the conventional art described above, since the period of performing the stratified spark-ignition combustion is determined based on a time length from the start of switching the combustion mode or a combustion cycle, the stratified spark-ignition combustion may continue even in a situation where the SI combustion or the HCCI combustion can stably be performed, which leaves space for improving the emission performance and the fuel consumption.